From what I've seen, adult baby fetish stuff either focuses on the humiliation aspect (i.e., forcing yourself to eat your own cummies) or on fetishizing the parent/infant relationship. Plushie fetishists seem to be more interested in a 'typical' relationship structure, they just build the 'boyfriend/girlfriend' structure around a plush doll. I know there was the one guy who acted like his plushie was a dominatrix, but that felt more like it was a side thing. Like, he's a guy who would like BDSM regardless, and just happened to also only be interested in fucking a stuffed animal.
If I was a psych major I'd totally want to do a study on these guys. Even if it meant I'd have to spend the better part of a semester immersed in plushie fuckers. Mechanically, there's nothing that makes being a plushie fucker and having a normal life mutually exclusive. It's not like you're trying to hide diapers from people at work or trying to figure out how to amputate your own arm. Hell, it's not even like you have to hide meeting up with your mistress or something. It could work that you're a normal guy with a normal job who goes home and fucks a stuffed tiger, but based on the evidence, no, that doesn't happen. You don't come out with everything else normal if you end up fucking plushies.
My guess would be that the sort of socially stunted person who doesn't want to give up the stuffed animals they slept with when they were little is more likely to have their first sexual experiences with a stuffed animal nearby--or with that stuffed animal. Some people might still grow out of it by that point, but those who remain socially retarded end up forming a romantic/emotional/sexual connection with the stuffed animals that were a part of their first sexual experiences.